Structure and dynamics of polymer-based glasses are studied below the glass-transition temperature with a combination of spectroscopy, theory, and polymer science. This approach operates at the interface of physical chemistry and material science to utilize molecular information to account for observed bulk properties. Nuclear magnetic resonance, the tool utilized in this investigation, probes structure and motion at the level of chemical groups which is then related to the macroscopic or bulk material properties of the multicomponent glasses. Local intermolecular structure in these amorphous materials is determined by carbon-13 spin diffusion experiments, while molecular reorientation is monitored with spin relaxation and line shape experiments. Rapid development of NMR experiments and instrumentation continues to offer new avenues for the study of solids which will provide a level of insight not attainable a few years ago. Quantitative interpretational models are developed which link the spectroscopic data to mechanical and thermal response as well as such technologically relevant materials behaviour as plasticization and permeability. The particular multicomponent glasses, polycarbonates and their mixtures, are selected for study as the result of their interesting and important materials properties. The interpretational efforts are also tied to the development of theory. The use of lattice models to interpret polymer-diluent structure and dynamics as well as mobilityof sorbed gas is a case in point. Simulation and modeling are used to assist in the development of physically sensible interpretational models.